In the field of printing, the most common type printer has been the printer which impacts against record media that is caused to be moved past a printing line or line of printing. As is well-known, the impact printing operation depends upon the movement of impact members, such as print hammers or wires or the like, which are typically moved by means of an electromechanical derived system and which system enables precise control of the impact members.
In the field of dot matrix printers, it has been quite common to provide a print head which has included therein a plurality of print wire actuators or solenoids arranged or grouped in a manner to drive the respective print wires a very short, precise distance from a rest or non-printing position to an impact or printing position. The print wires are generally either secured to or engaged by the solenoid plunger or armature which is caused to be moved such precise distance when the solenoid coil is energized and wherein the plunger or armature normally operates against the action of a return spring.
It has also been quite common to provide an arrangement or grouping of such solenoids in a circular configuration to take advantage of reduced space available in the manner of locating the print wires in that specific area between the solenoids and the front tip of the print head adjacent the record media. In this respect, the actuating ends of the print wires are positioned in accordance with the circular arrangement and the operating or working ends of the print wires are closely spaced in vertically-aligned manner adjacent the record media. The availability of narrow or compact actuators permits a narrower or smaller print head to be used and thereby reduces the width of the printer because of the reduced clearance at the ends of the print line. The print head can also be made shorter because the narrow actuators can be placed in side-by-side manner closer to the record media for a given amount of wire curvature.
In the wire matrix printer which is utilized for receipt and journal printing operation, the print head structure may be a multiple element type and horizontally disposed with the wire elements aligned in a vertical line and supported on a print head which is caused to be moved or driven in a horizontal direction for printing in line manner across the receipt or journal paper and wherein the drive elements or transducers may be positioned in a circular configuration with the respective wires leading to the front tip of the print head.
In the wire matrix printer which is utilized for business forms or like record media printing operation, the print head may be oriented in a manner wherein the nose of the print head is pointed downward for printing on the form, slip or like media while the carriage and print head are moved above and across the form or media in the horizontal direction.
In the case of a wire matrix printer which is utilized for form or multi-copy printing, the difference in thickness of the forms or copies may require some means or mechanism for adjusting the gap or the distance between the print head and the printer platen. It must also be appreciated and is well known that in view of variations in equipment and of energizing forces applied thereto, the impact for printing may vary and thereby result in different density images. It is desirable that the energy supplied to each print wire drive element be controlled in an overall arrangement wherein the density of each print image is substantially constant.
Representative documentation in the field of wire matrix print heads used for printing receipts, journals, forms or like record media includes U.S. Pat. No. 3,712,212, issued to J. Beery on Jan. 23, 1973, which discloses apparatus for printing characters with an impact intensity varying in accordance with the surface area of the character being printed. Current through the hammer driving solenoid coil is limited by resistors of different values which are controlled by electrical gates for selectively varying the amount of current supplied to the coil.
U.S. Pat. No. 3,866,533, issued to R. L. Gilbert et al. on Feb. 18, 1975, discloses impression control for an impact printer by changing the width of the pulse applied to the print hammers in accordance with the thickness of the forms being printed and in accordance with the voltage of the hammer energizing source to maintain a constant impact force for uniform print density.
U.S. Pat. No. 4,027,761, issued to R. S. Quaif on June 7, 1977, discloses a matrix print head impact energy control circuit wherein energy supplied to the solenoids is maintained constant notwithstanding variations in the power supply. Supply voltage and a reference voltage are coupled to a summing amplifier which is pulse width modulated to produce a pulsed hammer drive output having constant print energy. A single impact energy control circuit controls all of the print hammers.
U.S. Pat. No. 4,293,888, issued to V. D. McCarty on Oct. 6, 1981, discloses a print hammer drive circuit with compensation for voltage variation wherein the driving current level to a coil is detected and a timing circuit is initiated to control duration of application of maximum current.